The present invention discloses a method for analyzing ceramic materials to determine their impurity content. More specifically the present invention describes a Glow Discharge Mass Spectrometry (GDMS) analysis technique for determining trace impurity content in ceramic and ceramic-type materials such as alumina and silica.
Ceramic materials play an increasing role in meeting the needs of industry and society. Because of their high melting temperatures, thermal shock capabilities and resistance to harsh atmospheres, various ceramic products have found their way into such areas as automotive, lighting and space technologies. In order to minimize the chances of a part failing prematurely, the starting powders should be screened for contaminants, which may cause microscopic fracturing during forming operations or from mechanical stress during use. This quality control may be performed by various methods including spark source mass spectrometry, (SSMS), emission spectrography, inductively coupled plasma, spectrometry (ICP) or glow discharge mass spectrometry, (GDMS).
While emission spectrography does offer adequate sensitivity, it lacks accuracy, is time consuming and the photoplate may be difficult to interpret. SSMS offers similar sensitivity and better accuracy, but it lacks the resolution necessary to separate interference peaks from the peaks of interest. For example, the identification of the silicon dimmers from the iron and nickel peaks are not readily determined using SSMS because of isotopic interferences. Also, matrix effects of this technique can be severe, making it more standard dependent than GDMS. ICP offers good sensitivity and excellent accuracy, but sample preparation is difficult and time consuming for many ceramic materials, and great care must be taken not to volatilize any elements such as boron or silicon during the dissolution process with hydrofluoric acid.
In GDMS, the sample to be analyzed forms the cathode in a low pressure gas discharge. Argon is typically used as the gas. Positive gas ions are accelerated towards the cathode with energies of a few hundred electron volts thereby sputtering the sample. The sputtered neutral species diffuse through the discharge gas where some are ionized. The positive ions are extracted through a small slit and accelerated into a high resolution mass spectrometer for analysis.
The glow discharge produces a stable ion beam with few multicharged species and is therefore suited to producing consistent data. In contrast, the traditional spark ion source has poor ion stability and produces complex mass spectra which requires long integration times to optimize the sensitivity, commonly uses photographic plates for detection, as well as the need for a skilled operator to interpret the mass spectra on the photo plates.
While recently gaining in prominence, GDMS is an old analytical technique. Also, it is not the panacea for all elements. For example, potassium and calcium determinations at the low ppm range are not possible because of interferences from argon ions. Using a different discharge gas, such as xenon, reduces this problem, but at the expense of sensitivity. GDMS does offer excellent resolution (4000-10000 Daltons), straight forward sample preparation, and short analysis time.
Samples that are analyzed by GDMS must be conducting since they serve as one of the electrodes of a small hollow cathode cell. Therefore nonconducting material, such as insulating and ceramic materials must be mixed with a high purity conducting powder such as In(indium), Ga(gallium), or Ag(silver). For most insulating materials this procedure is quite satisfactory. However, it has been shown that materials such as silica and alumina cannot be run using the standard approach because the discharge (voltage and current) in the hollow cathode cell is not constant enough to allow for stable cell operation. This instability can be reduced if the sample to binder (silver, indium or gallium) ratio is reduced to 1 part Sample to 50 parts binder, or if the discharge parameters are extremely low, 0.2 mA. However, this stability is achieved at the expense of sensitivity, which now would be greater than 100 ppm for many elements.
The present invention describes a technique wherein ceramic materials that were previously not possible to analyze, such as alumina and silica, can be analyzed for impurities in the part-per-million range using GDMS.